1. Field of the Invention
The present invention relates to a honeycomb filter, and more particularly, it relates to a honeycomb filter which can suppress generation of cracks in plugged portions due to heat generated by burning of a PM during regeneration.
2. Description of the Related Art
In consideration of an impact on environment, there is an increasing need to remove, from an exhaust gas, a particulate matter included in the exhaust gas discharged from an internal combustion engine such as an engine for an automobile, an engine for a construction machine or an engine installed for an industrial machine, another combustion apparatus or the like. In particular, regulations on the removal of the particulate matter (referred to also as PM) discharged from a diesel engine tend to be strengthened worldwide. In such situations, a diesel particulate filter (DPF) for collecting and removing the PM has been gotten much attention.
As one embodiment of the DPF, there is a honeycomb filter comprising porous partition walls arranged to form a plurality of cells constituting through channels for a fluid, and predetermined cells each having one end opened as well as another end plugged and the remaining cells each having one end plugged as well as another end opened are alternately arranged. In this honeycomb filter, the fluid (the exhaust gas) which has flowed into the cells through opened one end of the predetermined cells penetrates through the partition walls to flow out of the partition walls as the penetrating fluid toward the remaining cells, and further flows out of the remaining cells through the opened another end thereof. When the exhaust gas penetrates through the partition walls, the PM included in the exhaust gas is collected and removed. In such a honeycomb filter (a wall flow type filter) having a structure in which the exhaust gas penetrates through the porous partition walls, the filter can cover a much filtering area. Therefore, the filter can reduce a filtering flow speed (a partition wall penetrating flow speed), and have a small pressure drop and a comparatively satisfactory particulate matter collecting efficiency.
However, the DPF to which this honeycomb filter is applied has problems as follows. When the PM is collected in a clean state, the state first shifts to a depth filtration state where the PM penetrates into pores of the porous partition walls to be collected therein, and a surface filtration state where the PM is collected on the surfaces of the partition walls. When the PM is deposited on the surfaces of the partition walls, the state next shifts to a cake filtration state where the PM forms a layer to perform a function of a filter. In such filtration processes, the PM is deposited in (the pores of) the partition walls in an initial depth filtration process. Therefore, a substantive porosity of the partition walls lowers immediately after start of the PM collection, and the flow speed of the exhaust gas penetrating through the partition walls lowers to make the pressure drop increase rapidly. This has caused deterioration of an engine performance.
It has been disclosed that to prevent the insides of the partition walls from being clogged with the PM, a membrane made of a porous material having a smaller average particle diameter than a porous material constituting the partition walls is formed on the surfaces of the partition walls on the side of inflow cells, to separate a filtering fluid from a filter cake containing fine particles (e.g., see Patent Documents 1 and 2). Moreover, a manufacturing method of a filter element provided with such a filter layer is disclosed (e.g., see Patent Document 3). It is to be noted that in the present description, these membranes or filter layers will be referred to as a collecting layer.
[Patent Document 1] JP-U-2607898
[Patent Document 2] JP-B-3261382
[Patent Document 3] JP-A-H10-249124